why would abigail admit she danced with the devil and wrote in his book?

Answers
1

Abigail needed to cover her tracks after being caught in her initial ritual with the cauldron. She has also laid fertile ground for her accusations to begin. Now publicly admitting that she has cavorted with the Devil, she can now begin implicating others and getting revenge on people she holds grudges against. She can also avoid blame by playing people against each other.